Electrical, molecular and behavioral effects of interictal spiking in the rat

Abstract

Objective Epilepsy is a disease characterized by chronic seizures, but is associated with significant comorbidities between seizures including cognitive impairments, hyperactivity, and depression. To study this interictal state, we characterized the electrical, molecular, and behavior effects of chronic, neocortical interictal spiking in rats. Methods A single injection of tetanus toxin into somatosensory cortex generated chronic interictal spiking measured by long-term video EEG monitoring and was correlated with motor activity. The cortical pattern of biomarker activation and the effects of blocking MAPK signaling on interictal spiking and behavior were determined. Results Interictal spiking in this model increases in frequency, size, and becomes repetitive over time, but is rarely associated with seizures. Interictal spiking was sufficient to produce the same molecular and cellular pattern of layer 2/3-specific CREB activation and plasticity gene induction as is seen in the human interictal state (Beaumont 2011, submitted). Increasing spike frequency was associated with hyperactivity, demonstrated by increased ambulatory activity and preferential circling toward the spiking hemisphere. Loud noises induced epileptic discharges, identical to spontaneous discharges. Treatment with a selective MAPK inhibitor prevented layer 2/3 CREB activation, reduced the frequency of epileptic discharges, and normalized behavioral abnormalities, but had no effect on seizures induced by electrical kindling. Interpretation These results provide insights into the development of interictal epileptic spiking, their relationship to behavior, and suggest that interictal and ictal activities utilize distinct molecular pathways. This model, that parallels recent observations in humans, will be useful to develop therapeutics against interictal spiking and its behavioral comorbidities.

Activity-Dependent Human Brain Coding/Noncoding Gene Regulatory Networks

“…Depolarization with supraphysiological concentrations of KCl has been used extensively as a means to study CREB activation and downstream transcription in neuronal cells in culture (Sheng et al 1990;Connolly and Kingsbury 2010). Figure 5A shows that, while a single treatment of these cells with 100 mM KCl leads only to transient CREB activation (CREB phosphorylation, detectable by the pCREB Ser106 antibody), repeated 5-min exposures with KCl separated by 2-hr intervals lead to more sustained CREB activation, similar to that observed in highly spiking human neocortex (Beaumont et al 2012) and in an animal model of frequent interictal spiking (Barkmeier et al 2012) (Figure 5B). …”

“…In fact, several of the genes induced at seizure-onset zones correlate precisely with interictal spiking rather than with seizure frequency (Rakhade et al 2007), suggesting that interictal spiking may be the driving force behind this altered expression pattern. Consistently, an animal model of interictal spiking without seizures was sufficient to produce neuronal layer-specific changes in these genes (Barkmeier et al 2012). Here we have focused on brain regions with different levels of interictal spiking to identify the relationships between coding and noncoding transcripts in the in vivo human brain.…”

“…Upstream of CREB activation, several known signaling pathways are rapidly activated in response to neuronal activity (Kandel 2001; reviewed in West et al 2002), including CaMKinase IV, protein kinase A, and MAPK. We have recently observed a pattern of transcriptional activation in human brain regions where seizures start that strongly implicates sustained MAPK/CREB activation and downstream coding gene activations that could underlie layerspecific changes in synaptic architecture that makes these regions prone to seizures (Rakhade et al 2005;Barkmeier et al 2012;Beaumont et al 2012).Given that human lncRNA genes tend to be less wellconserved than protein-coding genes, and can give rise to unique transcripts not found in other species, we sought out a uniquely human system to examine activity-dependent gene expression for both coding and noncoding RNAs using a pairwise comparison of human cortical regions displaying variable degrees of epileptic activities. These brain regions were removed as part of surgical treatment for intractable seizures.…”

“…Upstream of CREB activation, several known signaling pathways are rapidly activated in response to neuronal activity (Kandel 2001; reviewed in West et al 2002), including CaMKinase IV, protein kinase A, and MAPK. We have recently observed a pattern of transcriptional activation in human brain regions where seizures start that strongly implicates sustained MAPK/CREB activation and downstream coding gene activations that could underlie layerspecific changes in synaptic architecture that makes these regions prone to seizures (Rakhade et al 2005;Barkmeier et al 2012;Beaumont et al 2012).…”

Intracranial EEG analysis in tumor-related epilepsy: Evidence of distant epileptic abnormalities

“…Therefore, lower spike amplitude may be due to effects of the tumor cell infiltration effectively "diluting" the number of depolarizing neurons under the electrode or affecting layer-specific differences in neuronal networks as recently described in both human data and in a rat model of interictal spiking (Barkmeier et al, 2012a, Beaumont et al, 2012.…”